Observations of semidiurnal M 2 internal tidal parametric subharmonic instability in the northeastern South China Sea

Near-diurnal vertically-standing waves with high vertical wavenumbers k z were observed in the velocity and shear fi elds from a set of 75-d long ADCP moored in the northeastern South China Sea(SCS)away from the“critical”latitude of 28.8°.These enhanced near-diurnal internal waves followed a fortnightly spring-neap cycle.However,they always happened during semidiurnal spring tides rather than diurnal springs although strong diurnal internal tides with the fortnightly spring-neap cycle were prevailing,suggesting that they were generated via subharmonic instability(PSI)of dominant semidiurnal M 2 internal tides.When two semidiurnal internal tidal waves with opposite vertical propagation direction intersected,both semidiurnal subharmonic and super harmonic waves were largely intensifi ed.The observed maximum diurnal velocity amplitudes were up to 0.25 m/s.The kinetic energy and shear spectra further suggested that frequencies of daughter waves were not always perfectly equal to M 2/2.The superposition of two daughter waves with nearly equal frequencies and nearly opposite k z in a PSI-triad leaded to the vertically-standing waves.

Observations of near-inertial waves induced by parametric subharmonic instability

Near-inertial waves(NIWs), which can be generated by wind or the parametric subharmonic instability(PSI) of internal tides, are common in the South China Sea(SCS). Moored current observations from the northern SCS have revealed that the PSI of semidiurnal(D_2) internal tides is another source of NIWs. The objective of this study was to examine the energy variance in the PSI of D_2 tides. The PSI of D_2 internal tides generated NIWs and waves with frequencies around the difference frequency of D_2 and f. The observed NIWs induced by PSI could be distinguished clearly from those elicited by typhoon Krosa. Shortly after Krosa entered the SCS, NIWs began to intensify on the surface and they propagated downward over subsequent days. The near-inertial currents were damped quickly and they became relatively weak before the waves were reinforced beneath the mixed layer when wind stress was relatively weak. Rotation spectra indicated an energy peak at exactly the difference frequency D_2–f of the NIWs and D_2, indicating nonlinear wave-wave interaction among D_2, f, and D_2–f. Depth-time maps of band-pass fi ltered velocities of D_2 –f showed the waves amplifi ed when the NIWs were reinforced, and they intensifi ed at depths with strong D_2 tides. The energies of the NIWs and D_2 –f had high correlation with the D_2 tides. The PSI transferred energy of low-mode D_2 internal tides to high-mode NIWs and D_2–f waves. For the entire observational period, PSI reinforcement was observed only when mesoscale eddies emerged and when D_2 was in spring tide, revealing a close connection between mesoscale eddies and NIWs. Mesoscale eddies could increase the energy in the f-band by enhancing the PSI of D_2 internal tides. Thus, this represents another mechanism linking the energy of mesoscale eddies to that of NIWs.

南海西北部半日内潮参量次谐频不稳定生成的近惯性波研究

为了研究非临界纬度上参量次谐频不稳定(parametric subharmonic instability, PSI)过程生成的近惯性波(near-inertial wave, NIW),本文基于国家重点研发项目的准实时传输深海潜标资料,对内波速度谱、近惯性流速和动能、D_(2)-f(半日频减惯性频)流速和动能、半日内潮流速和动能以及混合的时间分布特征进行了研究分析。平板模型与双相干谱分析表明,南海西北部陆坡区上层海洋出现的非风生近惯性波是由半日内潮PSI过程生成的,同时也会生成D_(2)-f内波。在发生显著PSI过程的深度上,生成的近惯性能量与全日内潮能量相当,生成的D_(2)-f能量与半日内潮能量相当,表明PSI过程生成的近惯性波和D_(2)-f内波是相当重要的。细尺度参数化方法结果显示PSI过程期间混合出现明显增强,但其增加的大小是中度的。

Internal Waves Generated by Tidal Flows over a Triangular Ridge with Critical Slopes

Direct numerical simulations are performed to investigate the generation of internal waves by tide-topography interaction in a lab-scale model.The bottom topography is a triangular ridge with two critical slopes.With increasing tidal forcing,subharmonic instabilities are identified,which cause internal wave beams to become unstable and turbulent.Kinetic energy densities in the upward going beams from the ridge top are stronger than those from the ridge bottom,whereas the reverse is true for the energy flux.This disparity between energy and energy flux is due to the existence of strong pressure disturbances near the ridge bottom.On each side of the critical ridge,there exists an amphidromic point,from which internal wave beams are emitted in opposite directions.The calculated energy conversion rate scales linearly with the square of the forcing amplitude and agrees within 13%of theoretical prediction,even when turbulence occurs.The fraction of radiated baroclinic energy becomes saturated in the range of low excursion parameter considered,which agrees with the behavior in large-scale systems wherein mixing parameterizations must be used.The present work enriches the studies on the generation of internal waves over a critical triangular ridge.